mobile internet speed while on train

[number42]

So i have this question about my mobile internet;

Does it slow down when my train is moving? Right now, i'm in the train on my way home, and using the internet on my laptop with a bluetooth connection to my phone. My phone only supports GPRS (i know...) so its about 5kbyte/s according to the specs.

But in october one of my subjects was relativity theory, and in january i had a subject about wave functions. They both had paragraphs about what happens with an electromagnetic wave when you are moving in relation to the inertial where the wave is coming from. The waves either stretch or compress, depending on your position in relation to the sender.

So how does my phone still understand what the waves mean? I mean, lets say an incoming high energy burst is a 1 and no energy is a 0. It has to measure the time it takes for the bursts to come in, right? So if the waves are stretched by a factor 1.5, wouldnt my phone freak out and show a bunch of errors?

Anyone knows how these systems work, and how they pull it off to work at 200mph (my current velocity)?

 

[Corny]

you are probably not moving fast enough for this to have an effect

the bigger problem is that the phone has to switch stations quite frequently. that's why some trains (in germany at least) have signal extenders/amplifiers.

 

[looseliam]

It's just the doppler effect.

I would imagine the phone isn't concerned with the frequency of the packets, rather the data contained.

 

[T-Rexx]

you are probably not moving fast enough for this to have an effect

I know you have high speed trains in Europe, but that would be incredible!


Corny is correct, of course. The problem is that the phone has to switch communication towers frequently, which requires constant re-routing of the connection between the web sites you are surfing and your phone on the train. That slows down your exchange of information with the web considerably, even though the speed of connection between your phone and the nearest cell tower remains constant.

Consider, for example, how the United Nations has defined a 4G wireless data connection. It is one which gives you 1 GB/s speed when stationary, and 100 MB/s when moving. A mobile speed of only 10% of the stationary speed is thought to be realistic. (BTW, it happens that not one of the wireless data services currently marketed in the USA as 4G currently meets this criteria).

 

[number42]

Right, now i get it. The point where i got mixed up was with the doppler effect, as looseliam said. But radiowaves go at lightspeed so it wouldnt be much of a change.

So if i would ride around a cell tower at 400mph, so i stay within it's zone so i wont have to change tower, will my internet speed be the same as stationary? Is the tower switching the ONLY problem?

 

[T-Rexx]

Right, now i get it. The point where i got mixed up was with the doppler effect, as looseliam said. But radiowaves go at lightspeed so it wouldnt be much of a change.

The Doppler effect is dependent on the speed of the wave through the medium, as well as your speed relative to the wave-emitting source. But radio waves travel at the speed of light, which is so fast it essentially eliminates any effect of your speed, unless you are on a lightspeed spaceship.

Wikipedia gives the formula for calculating the difference between emitted frequency of the wave and observed frequency: http://en.wikipedia.org/wiki/Doppler_effect

For the Doppler effect even to be noticeable, you would have to be travelling at a reasonable fraction of the speed of light. But light travels at 300,000 km/s; even 5% of that would be 15,000 km/s (=54,000,000 km/hr). Eurostar is impressive, but it's not that impressive!

So if i would ride around a cell tower at 400mph, so i stay within it's zone so i wont have to change tower, will my internet speed be the same as stationary? Is the tower switching the ONLY problem?

Yes. If your train was circling the cell tower, you would appreciate the same speeds as if you were stationary.

Italy has some very long train tunnels through the mountains there. I have noticed that when the train enters the tunnels, cell phone conversations by people on the train do not get dropped. This tells me that they must have a cell phone antenna running the length of the tunnel. It has occurred to me that if you could do that for the length of the track (even outside of tunnels), you could speed up your data connection, since you would essentially be staying within a single cell (albeit a long and very skinny one).

 

[number42]

Wouldnt it be easier to just make antenna's inside the train? Trains here are going to be outfitted with mobile network extenders. So it'd be a cell on the move. Instead of a giant antenna next to the track.

 

[T-Rexx]

The antenna on the train would still be moving. So it would still be switching from communications tower to communications tower as the train moved.

It is the switching from cell to cell that slows down mobile communications.

For a long train ride, I doubt that it would be practical to maintain a single cell for the length of the track. But, you could probably limit the switching from one cell to another, by designing train-specific cells that run alongside the track.

 

[Corny]

. But radio waves travel at the speed of light, which is so fast it essentially eliminates any effect of your speed, unless you are on a lightspeed spaceship.
[..]
For the Doppler effect even to be noticeable, you would have to be travelling at a reasonable fraction of the speed of light. But light travels at 300,000 km/s; even 5% of that would be 15,000 km/s (=54,000,000 km/hr).

you are probably not moving fast enough for this to have an effect

8)

Italy has some very long train tunnels through the mountains there. I have noticed that when the train enters the tunnels, cell phone conversations by people on the train do not get dropped. This tells me that they must have a cell phone antenna running the length of the tunnel. It has occurred to me that if you could do that for the length of the track (even outside of tunnels), you could speed up your data connection, since you would essentially be staying within a single cell (albeit a long and very skinny one).

Wouldnt it be easier to just make antenna's inside the train? Trains here are going to be outfitted with mobile network extenders. So it'd be a cell on the move. Instead of a giant antenna next to the track.

For a long train ride, I doubt that it would be practical to maintain a single cell for the length of the track. But, you could probably limit the switching from one cell to another, by designing train-specific cells that run alongside the track.

as I said

the bigger problem is that the phone has to switch stations quite frequently. that's why some trains (in germany at least) have signal extenders/amplifiers.

at some tracks (and tunnels) they have extra antennas, but it even helps when those are not present. the train has a huge antenna that makes the "far station" available earlier. i believe some tubes also have the "train as a station" variant, however this results in you only having a certain network (or in big cities two) available.